1. Field of the Invention
This invention relates generally to semiconductor photodetectors and, more specifically, to a resonant semiconductor photodetector that is able to generate an electrical output for a wide range of electromagnetic frequencies using a waveguide reflector and a semiconductor reflector to create a resonance of light within the photodetector active area.
2. Discussion of the Related Art
High frequency photodetectors, such as PIN photodiodes, that are used in a variety of systems for the transfer of light as a primary means of transferring information are known in the art. These systems are especially needed for high-speed communication systems, such as automatic teller machines, computer network systems, and multimedia applications.
Photodetectors are used to convert optical energy into an electrical energy. A photodiode is typically used for high-speed applications. In high-speed applications, the speed and the responsivity of the photodetector are critical. Although fiber optic cable can transmit at speeds greater than 100 GHz, current technology photodetectors are limited to 45-60 GHz bandwidths. With the current explosion of multimedia technologies and applications, such as the Internet, the telecommunications industry will require higher bandwidth systems, such as optical systems with high speed photodiodes.
In the typical photodiode, an active semiconductor material generates an electrical current by the photogenerated electrons within the active material. Responsivity and speed are two variables that are often used to determine the performance of photodetectors. Responsivity is the measure of the effectiveness of a device in converting incident light to an output current. Speed is the measure of how quickly an output of the device changes in response to a change in the input to the taxi device. For a photodiode to be effective in high-speed communication applications, it must have both a high responsivity and a high speed. Current high speed photodiodes typically have a responsivity of 0.2-0.4 amps/watt and a top end speed of 45-60 GHz. To increase the responsivity of a photodiode, the thickness of the active area is often increased so as to increase the quantum efficiency, thus creating more output current. This creates a problem, however, because the thicker the active area, the longer the transmit time, which decreases the speed.
Current high speed photodiode design incorporates a tradeoff between quantum efficiency and bandwidth.
Most communication applications that involve photodiodes also require an optical coupling device for guiding the light to the photodiode active area. Since the requirements of the optical coupling device are to deliver the incident light to a relatively small area, typically there are a number of components and materials that are required to carry out this task. Due to the difference of materials and the number of optical components that are used in the optical coupling device, there tends to be a high optical loss in the coupling device that degrades the overall performance of the photodiode.
State of the art optical communication systems have carriers of very high frequency that require the use of high speed, high responsivity photodetectors. As the demand for more information increases, so will the demand that communication systems be able to transmit more information, which will in turn require high-speed high responsivity photodetectors. The known photodetectors for high frequency applications are limited by having a low responsivity and a limited high-end frequency response. It has been recognized that the effectiveness of a communications system could be increased by providing a photodetector that employs multiple reflections between a waveguide and reflectors to produce a high response and high-speed photodetector.
It is an object of the present invention to provide a resonant photodetector that provides for an increased responsivity and speed, as well as providing other improvements, over the known photodetectors, to improve the performance of the communication process.
In accordance with the teachings of the present invention, a resonant photodetector assembly is disclosed that utilizes multiple reflections within a photodetector to convert an optical signal to a corresponding electrical output. The resonant photodetector assembly includes a waveguide, a first reflector, a photodetector, a second reflector and supporting structure.
The waveguide provides a path to direct light to the photodetector from a light source. The waveguide includes a first end that provides a connection point to a light source. This single element reduces the number of optical components required to direct the light to the photodetector. The waveguide further includes cladding layers that refract the light to prevent the light from escaping from the waveguide while propagating towards the photodetector. The first reflector and the second reflector provide for multiple reflections of the light propagating through the photodetector. This use of multiple reflections within the photodetector allows for a thinner active area to be used in the photodetector. A thin active area is required for high speed operation, the use of multiple reflections through the active layer provides high speed while retaining or increasing the efficiency of operation.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.